Learning C# on Mac

I have played around with C# a few times and I would like to expand my knowledge so that I can confidently add it to my resume and apply for jobs. I have Xamarin Studio on my Mac and have played lightly with the Unity Engine, but I was wondering if there are any good books or websites that provide guidance on open source C#.

Also, assuming that I become proficient enough to get to a job using C#, how big of a transition would I have to make going from open source to MSDN?